1. Field of the Invention
The invention relates generally to methods for assuring performance of photomasks employed for fabricating microelectronic products. More particularly, the invention relates to methods for assuring performance of phase shift photomasks employed for fabricating microelectronic products.
2. Description of the Related Art
Photolithographic processes and methods are integral to the fabrication of microelectronic products. They are employed for defining patterned photoresist mask layers that are employed within various microelectronic fabrication processes. The microelectronic fabrication processes may include, but are not limited to etch processes, deposition processes and ion implantation processes.
As microelectronic product feature dimensions have decreased, it has become increasingly difficult to form patterned photoresist layers with enhanced resolution while employing conventional photomasks and conventional actinic radiation sources. Phase shift photomask constructions have arisen as a result of the need for forming such patterned photoresist layers with enhanced resolution. Phase shift photomasks differ from conventional photomasks insofar as phase shift photomasks provide for a phase shifting of actinic radiation such as to provide a higher resolution (i.e., sharper) actinic radiation intensity peak transmitted through a phase shift photomask. In turn, this provides a correspondingly enhanced resolution of a patterned photoresist layer formed employing a phase shift photomask.
Various phase shift photomask constructions are known. A particularly common phase shift photomask construction is an attenuated phase shift photomask construction. It employs transparent areas of a transparent substrate that are separated by masked regions of the substrate formed from an incompletely opaque material layer. Typically, the incompletely opaque material layer has a transmittance on the order of several percent. The thickness of the incompletely opaque material layer is selected such as to provide for a phase shift of actinic radiation when employing the attenuated phase shift photomask.
While attenuated phase shift photomasks provide particular advantage when employed within photolithographic methods, they are nonetheless not entirely without problems. In that regard, it is particularly important when employing attenuated phase shift photomasks that a transmittance of an incompletely opaque material layer be within a generally narrow range. At a transmittance level below the narrow range, enhanced resolution of transmitted actinic radiation is not realized. At a transmittance level above the narrow range, a side lobe effect of the transmitted actinic radiation may occur. The side lobe effect yields an undesirable inhomogeneous photoexposure and development of a patterned photoresist layer.
Desirable are methods for assuring optimal performance of attenuated phase shift photomasks. The invention is directed towards the foregoing object.